1. Field of the Invention
The present invention relates to a wireless tag reader, a wireless tag system, a computer readable medium and a wireless communication method.
2. Related Art
In recent years, introduction of wireless tag systems has increased to efficiently manage commodities. In the wireless tag system, RF-ID tags respectively embedded in commodities and a reader/writer which reads out data from the RF-ID tags are used. If responses are sent simultaneously from a plurality of RF-ID tags when the reader/writer reads out data, collision between data occurs.
As conventional anti-collision technique, the ALOHA system and binary search are prominent.
The ALOHA system will now be described. If the reader/writer detects collision in response signals to a query sent to RF-ID tags, the reader/writer sends queries with conditions narrowed down in order from the most significant bit (MSB) of tag IDs in RF-ID tags. This is repeated until the condition that only one of RF-ID tags causing collision reacts is satisfied. This ALOHA system has a feature that a logic circuit to be mounted on the RF-ID tag is simple. In this system, however, a procedure of narrowing down using a plurality of steps is conducted until simultaneously reacting RF-ID tags are narrowed down to one tag. This results in a drawback that it takes much time in reading if the number of steps increases.
On the other hand, the binary search can be applied to an encoding scheme in which when RF-ID tags having different tag IDs simultaneously response, only a difference portion (a portion having different bit values) in the tag ID can be detected by the reader/writer as a collision signal and a common portion (a portion having the same bit value) can be read out as it is. For example, in the case of Manchester coding, collision can be detected when 0 and 1 are transmitted simultaneously. Also in the case where the binary search is used, there is a drawback that it takes much time in reading if the number of steps increases.
Besides, there is Japanese Patent Application Laid-Open Publication (JP-A) No. 2004-62665 as a document showing the conventional anti-collision technique.
An anti-collision mechanism described in JP-A-2004-62665 implements an anti-collision function using a simple algorithm of shifting a response slot start position of an RF-ID tag when collision has occurred, on the basis of the conventional algorithm (JIS-X6323-3, ISO/IEC15693-3). If simply the slot is shifted, there is a possibility that collision occurs again. However, in the JP-A-2004-62665, it is described that if there is a sufficient ID space (56 bits or more in the description of JP-A-2004-62665) the possibility of recurrence of collision is low and a tag ID can be acquired after several attempts.
Even if this technique described in JP-A-2004-62665 is used, however, only one tag ID can be acquired after several attempts. Therefore, there is a problem that a lot of time is needed to acquire tag IDs from all RF-ID tags in a detectable area by the reader/writer, i.e., to detect all RF-ID tags existing in the detectable area.